The compression ratio of a reciprocating piston internal combustion engine E is used to describe the ratio of the volume of the entire cylinder chamber to the volume of the compression chamber. Through an increase in the compression ratio, the efficiency of the reciprocating piston internal combustion engine may be increased and the fuel consumption my thus be reduced overall. With reciprocating piston internal combustion engines with external ignition, their knocking tendency however increases with the increase of the compression ratio in full-load operation. The knocking is an uncontrolled self-ignition of the air-fuel mixture. On the other hand, the compression ratio during partial-load operation, during which the filling is lower, could be increased for the improvement of the respective partial-load efficiency, without the previously mentioned knocking occurring. The result is thus that it is expedient to operate the reciprocating piston internal combustion engine in partial-load operation with a relatively high compression ratio and in full-load operation with a compression ratio reduced towards it.
A change of the compression ratio is also particularly advantageous for loaded reciprocating piston internal combustion engines with external ignition, as a lower compression ratio is stipulated with them in view of the load, whereby the compression must be increased to improve the thermodynamic efficiency in unfavorable areas of a respective engine map. There is furthermore the possibility to generally change the compression ratio depending on more operating parameters of the reciprocating piston internal combustion engine, for example like with running conditions of motor vehicles, operating points of the internal combustion engine, signals of a knock sensor, emission values, etc.
Devices are known from the state of the art, among other things, which perform an adjustment in distance between a crankpin, a crankshaft, and a piston pin. It thereby pertains to different devices, namely to such that are used to change the position of the piston as opposed to the connecting rod, or to such that perform a change in position of the connecting rod as opposed to the crankshaft.
A device for changing a compression ratio of a reciprocating piston internal combustion engine in the type described in the generic claim of patent claim 1 is known from DE 197 03 948 A1. According to that, a connecting rod divided in the area of its connecting rod bearing eye accommodates an eccentric bushing through the bore of the connecting rod bearing eye. It is guided to an inside wall of the connecting rod bearing eye with its cylindrical outer circumferential surface and is supposed to be formed as a plain bearing. On its inside circumferential surface that is eccentrically offset to the outer circumferential surface, the bushing is guided sliding on a crankpin of a crankshaft of the reciprocating piston internal combustion engine. A rotation of the bushing in the bore of the connecting rod bearing eye shall be automatic. This movement of the eccentric bushing shall be triggered by outside forces resulting from the rotation of the crankshaft, respectively the connecting rod movement. The eccentric bushing is furthermore provided with flanges on its ends, which overlap the connecting rod bearing eye and have locking recesses offset by 180° in peripheral direction. A locking member intended on the connecting rod is activated through a cam segment in such a way that it optionally engages in the one or the other locking recess after a respective twist of the bushing.